Anilines are important industrial chemicals of great commercial usefulness and are widely used in various fields of applications such as rubber vulcanization accelerators, antioxidants, dyes, intermediate dyes, etc., or starting materials for aniline resins. In recent years, moreover, new uses as starting materials for photographic chemicals, agricultural chemicals and medicines of aniline derivatives, for example, such compounds as toluidine, cumidine, methyl cumidine, xylidine, etc., are increasingly expanding.
Such anilines as mentioned above have heretofore been prepared by processes involving 1 catalytic reduction of aromatic nitro compounds, 2 reaction of aromatic halides with ammonia water at high temperatures under pressure, or 3 reaction of phenols with ammonia.
In the above-mentioned process 1 involving the use of the nitrocompounds, however, there is involved such problem that because of necessitating large amounts of sulfuric acid and nitric acid as nitration agents, large amounts of alkali substances such as sodium hydroxide are needed in the neutralization step and, moreover, large amounts of waste water containing salts in high concentrations are produced. In addition thereto, there is involved such problem that, as pointed out in Japanese Patent L-O-P Pubin. No.67229/1973, nitrogen oxide gas is generated during the course of operation to prepare the nitro compounds, whereby air pollution with said nitrogen oxide gas is brought about.
The process 2 relying on the use of aromatic halides has such a fundamental problem that expensive corrosion-resistant equipments should be used for practicing said process because chlorine that is very high in corrosive properties has to be used therefor. Notwithstanding that the reaction of chlorobenzene with ammonia is carried out under high temperature and pressure circumstances, moreover, it has been pointed out that desired anilines are obtained in low yields, and under the present conditions this process is not practically applied to aromatic halides except for p-nitrochlorobenzene having a nitro group at the para-position.
From the reasons cited above, the process 3 relying on reaction of phenols with ammonia has become of major interest lately, and at present this process is coming to take a leading figure in the processes for preparation of anilines. That is, according to this process the desired anilines can be prepared by simply passing phenols and ammonia through fixed bed catalysts, and hence there are observed such excellent advantages that no problem of air pollution due to nitrogen oxide gas arises, no large amount of waste water are produced and the operational process can be markedly simplified.
As a typical example of the preparation of anilines by reaction of phenols with ammonia, there may be mentioned a process as disclosed in Japanese Patent Publication No.23571/1967. According to the process for preparing aminobenzene disclosed in the above-cited patent publication, aminobenzene such as aniline is prepared by reacting hydroxybenzene such as phenol with an amination agent at a temperature of 300.degree.-600.degree. C. by using a catalyst selected from the group consisting of silica-alumina, zirconia-alumina, titania-alumina, zirconia-silica phosphate and tungsten oxide. In this connection, this patent publication teaches that because of low activity exhibited in such amination reaction as mentioned above, weakly acidic solid acid such as commercially available-alumina catalyst is insufficient as a catalyst for use in said amination reaction, whereas silica-alumina catalyst which is a strongly acidic solid acid comprising silica or alumina in an amount of 10-20% of the weight of the catalyst is particularly excellent as a catalyst for use in the amination reaction.
However, where the strongly acidic solid acid catalyst such as silica-alumina catalyst is used in the amination reaction, there is such problem that undesirable side reaction such as decomposition of aniline being formed or formation of resinous substance as by product takes place though the initial activity of the amination reaction is high. Furthermore, there is encountered such a fatal problem that deterioration of the catalyst rapidly proceeds when such resinous substance attaches to the catalyst surface to cover the active site of the catalyst. On that account, it was necessary to effect frequently a catalyst regeneration operation.
An attempt to solve such problems as mentioned above is disclosed in Japanese Patent L-O-P Publn. No.67229/1973, which teaches that the reaction of phenols with an amination agent is carried out by using a catalyst which is weak in acid strength in comparison with the above-mentioned silica-alumina catalyst (pKa -8.0), that is, titania-zirconia and titania-silica catalysts which are solid acid catalyst, the acid value of which is distributed in the range of from -5.6 to -3.0 in terms of pKa. Even when such catalysts are used, however, the reaction temperature employed must be elevated to such high temperature as ranging from 400.degree. to 500.degree. C. in order to accomplish an effective amination reaction. At this elevated temperature, however, decomposition of ammonia which is the amination agent, that is, NH.sub.3 .fwdarw. 1/2N.sub.2 +3/2H.sub.2, is accelerated, whereby deterioration of the reaction apparatus due to nascent nitrogen at the time when it is generated. Thus, the process thus taught still involves such problem that a lifetime of the reaction apparatus used therefor is markedly shortened.
Furthermore, a sharp lowering of catalyst activity is observed during operation continued for only about 40 hours, and it is difficult to carry out this process on an industrial scale.
Besides the foregoing, Japanese Patent L-O-P Publn. No.23052/1971 discloses a process for amination of phenols using catalysts comprising a combination of dehydrated solid acid catalysts and hydrogenated catalysts, and Japanese Patent L-O-P publn. No.23053/1971 also discloses the amination of phenols by using catalysis comprising a combination of alumina or silica and an oxide selected from the group consisting of magnesia, boria and thoria. In each of these processes, however, an improvement made is barely prolongation of duration of the catalyst activity to 50-100 hours, and no solution of the catalyst deterioration problem is made at all.
In the manner now described, the known processes for the preparation of anilines by amination of phenols all require the employment of high temperature higher than 400.degree. C. in order to carry out the amination reaction efficiently. On that account, there were such fatal problems that the apparatus used therefor deteriorates due to nascent nitrogen generated by the decomposition of ammonia which is the amination agent, and that the catalyst regeneration operation is frequently necessary because the catalyst deterioration takes place due to contamination of the catalyst surface with resinous substance formed by decomposition of anilines being formed, deposition on the catalyst surface of carbonaceous substance formed by decomposition of organic materials or the like, thereby the catalyst activity decreases in a short period of time.